In order to process the incident data, it is indispensable to reconstitute the emission tempo, and it is the role of the extraction circuit to produce a clock signal whose frequency is identical to the emission tempo and which moreover has a phase which is correctly defined with respect to the transitions of the binary data received. The question of the phase is particularly important since it would serve no purpose to reconstitute a clock signal having the desired exact frequency, that of emission, but whose transitions coincided with the transitions of the data or were too close to the latter: it would not be possible to sample the data with such a clock. In practice, it is sought to reconstitute a clock signal having a rising edge set substantially in the medium of the interval between the theoretical instants at which the binary data transitions may occur.
In what follows, it will be considered that the frequency that is to be reconstituted is the frequency at which the data were emitted, so as to simplify the explanations; however, the invention applies generally to the reconstitution of a clock frequency present in the data received, independently of the frequency emitted, in the cases where there is a shift (due to a Doppler effect notably) between the frequency received and the frequency emitted.
Circuits for reconstituting clock signals from incident binary data have already been proposed in the prior art. Examples thereof will be found in the following publications:                US 2001/0043086; this document makes a digital measurement of a phase error and generates pulses of variable width proportional to the phase error; this principle of digital measurement prevents operation at very high frequency;        U.S. Pat. No. 5,124,669; this document also describes a digital measurement        U.S. Pat. No. 5,760,653; uses a clock with triangular waveform, difficult to implement at very high frequency; it also uses a sample-and-hold unit to control an oscillator with voltage-controlled frequency, so that this oscillator sees a control voltage vary steeply at each sampling;        JP 63094731; this document describes a system in which the incident edges of the incoming data are transformed into Gaussian forms, thereby tending to lose the information about the precise instant of the binary transitions;        U.S. Pat. No. 6,542,041; this document operates with digital phase detectors providing variable pulse widths; it does not lend itself to a high input data rate.        